The present invention relates to a nitride-based semiconductor laser device and a method for the production thereof, and more specifically to a nitride-based semiconductor laser device having an operation voltage controlled to be a desired value and having excellent lateral mode stability, and a method for the production of a nitride-based semiconductor laser device having an operation voltage controlled to be a desired value and having excellent lateral mode stability, in which the production process is simplified.
A GaN-based semiconductor laser device having a stacked structure of GaN-based compound semiconductor layers formed on a sapphire substrate or a GaN substrate is evoking much interest as a light-emitting device that emits light in a short wavelength region from an ultraviolet region to green.
The constitution of a GaN-based semiconductor laser device 100 disclosed in JP-A-2000-196201 will be explained below with reference to FIG. 10 showing a schematic partial cross-sectional view of a conventional index-guide type GaN-based semiconductor laser device.
The GaN-based semiconductor laser device 100 disclosed in JP-A-2000-196201 has a stacked structure in which, on a substrate 12 made, for example, of a sapphire substrate having a c-surface as a main surface, a first contacting layer 14 made of n-type GaN, a first cladding layer 16 made of n-type AlGaN, a first light-guiding layer 18 made of n-type InGaN, an active layer 20 having a multiple quantum well structure of GaN/InGaN, a degradation-preventing layer 21 made of AlGaN for preventing the degradation of the active layer 20, a second light-guiding layer 22 made of p-type InGaN, a second cladding layer 24 made of p-type AlGaN and a second contacting layer 26 made of p-type GaN are consecutively stacked. There are many cases where a buffer layer (not shown) made of GaN is grown on the substrate 12 at a low temperature, a substratum layer (not shown) made of GaN is laterally grown on the buffer layer, and then, the first contacting layer 14 is grown. There are also some cases where the first light-guiding layer 18 and the second light-guiding layer 22 are not provided, nor is the degradation-preventing layer 21 provided.
The upper layer 24B of the second cladding layer 24 and the second contacting layer 26 have, for example, a ridge structure extending unidirectionally in the form of a stripe. Further, part of the first contacting layer 14, the first cladding layer 16, the first light-guiding layer 18, the active layer 20, the degradation-preventing layer 21, the second light-guiding layer 22 and the lower layer 24A of the second cladding layer 24 have, for example, a mesa structure extending in the form of a stripe and in the same direction as the extending direction of the ridge structure. That is, the thus-structured GaN-based semiconductor laser device 100 satisfies Wxe2x80x21 greater than Wxe2x80x22 wherein Wxe2x80x21 is a width of the mesa structure and Wxe2x80x22 is a width of the ridge structure.
The ridge structure, the mesa structure and portions of the first contacting layer 14 positioned on both sides of the mesa structure are covered with a protection layer 28 made of SiO2 except for a second opening portion 28A formed on the topmost surface of the ridge structure (i.e., top surface of the second contacting layer 26) and a first opening portion 28B formed on part of the first contacting layer 14. On the second contacting layer 26 positioned in a bottom of the second opening portion 28A, a second electrode 30 having a multi-layered structure of Ti/Au (Ti forms a lower layer and Au forms an upper layer) is provided as an ohmic contact electrode. In the explanation of the multi-layered structure, a material before xe2x80x9c/xe2x80x9d forms a lower layer and a material after xe2x80x9c/xe2x80x9d forms an upper layer, and xe2x80x9c/xe2x80x9d will be used in this sense hereinafter. Further, on the first contacting layer 14 positioned in a bottom of the first opening portion 28B, a first electrode 32 having a multi-layered structure of Ti/Al is provided as an ohmic contact electrode. In addition, provided on the second electrode 30 and the first electrode 32 are a second pad electrode 34 and a first pad electrode 36 that are electrically connected to the second electrode 30 and the first electrode 32, respectively, as leading electrodes. The second pad electrode 34 extends from the second electrode 30 to the top surface of the protection layer 28.
In the above-structured GaN-based semiconductor laser device 100 disclosed in JP-A-2000-196201, the upper layer 24B of the second cladding layer 24 and the second contacting layer 26 have the ridge structure, so that the current passage of electric current injected is limited to decrease the operation current, and that the lateral mode is controlled by means of an effective refractive index difference xcex94n in a lateral direction. The effective refractive index difference xcex94n refers to a difference (xcex94n=nEFF1xe2x88x92nEFF2) between an effective refractive index nEFF1 obtained by measurement along the line Axe2x80x94A in FIG. 10 and an effective refractive index nEFF2 obtained by measurement along the line Bxe2x80x94B in FIG. 10.
The above GaN-based semiconductor laser device disclosed in JP-A-2000-196201 has the following problems.
The first problem is that the operation voltage of the GaN-based semiconductor laser device 100 comes to be higher than a desired value or a designed value.
The second problem is as follows. The lateral mode is controlled by means of the effective refractive index difference xcex94n in a lateral direction. However, it is difficult to increase the thickness of the upper layer 24B of the second cladding layer 24 and it is difficult to decrease the thickness of the lower layer 24A of the second cladding layer 24, so that the effective refractive index difference xcex94n in a lateral direction is small, and that the stability of the lateral mode is therefore poor. When the upper layer 24B of the second cladding layer 24 is increased in thickness and when the lower layer 24A thereof is decreased in thickness, leak current may flow through the protection layer 28 and the lower layer 24A of the second cladding layer 24.
The third problem is that the process which follows the formation of the stacked structure of GaN-based epitaxial growth layers is complicated and includes many steps, so that it is difficult to improve the productivity. After the formation of the stacked structure, for example, the process requires the steps of forming an etching mask made of SiO2, etching the second contacting layer 26 and further etching an upper portion of the second cladding layer 24 to form the ridge structure; the steps of forming a ZrO2 film on the entire surface and removing the ZrO2 film on the etching mask by removing the etching mask to expose the second contacting layer 26; the steps of forming an etching mask made of SiO2 again on the top surface the ridge structure, etching the lower layer of the second cladding layer and each layer positioned below said layer in the stacked structure to form the mesa structure, further, exposing the first contacting layer 14 and then removing the etching mask; and the step of forming the second electrode 30 on the exposed second contacting layer 26.
JP-A-2000-307184 discloses another method of producing a GaN-based semiconductor laser device. In the second embodiment of the method of producing a GaN-based semiconductor laser device disclosed in the above JP-A-2000-307184, after the formation of a stacked structure of GaN-based epitaxial growth layers, first, the stacked structure is etched to form a mesa structure. Then, a protection layer is formed on the entire surface, an opening portion is formed in the protection layer, an second electrode is formed on the top surface of the second contacting layer positioned in a bottom portion of the opening portion, and then the protection layer is removed. Then, while using the second electrode as an etching mask, the second contacting layer and part of the second cladding layer are etched to form a ridge structure. Then, an insulating layer is formed on the entire surface, and the insulating layer on the second electrode is removed to expose the top surface of the second electrode.
In the above method of producing a GaN-based semiconductor laser device disclosed in JP-A-2000-307184, first, the stacked structure is etched to form the mesa structure. In this case, the top surface of the second contacting layer that is to be a contact surface to the second electrode may be contaminated. Further, there is involved a problem that it is difficult to form a thick insulating layer on both side surfaces of the upper layer of the second cladding layer having the ridge structure.
It is therefore an object of the present invention to provide a nitride-based semiconductor laser device that operates at a low voltage and has excellent lateral mode stability, and a method for producing a nitride-based semiconductor laser device in which the above nitride-based semiconductor laser device can be produced by a process having steps decreased in number.
The present inventors have made diligent studies to overcome the first problem. The studies have revealed the following. The top surface of the second contacting layer 26 (contact surface to the second electrode) is contaminated, and, as a result, the contact resistance between the second contacting layer and the second electrode 30 increases, so that the operation voltage increases. It has been found that the contamination of the top surface of the second contacting layer 26 (contact surface to the second electrode) is caused by the existence of the step of forming the mesa structure and some other steps between exposure of the second contacting layer 26 and formation of the second electrode 30 on the second contacting layer 26, concerning the third problem. The following has been further found. The second electrode 30 and the second contacting layer 26 are liable to undergo displacement each other, and as a result, the contact area between the second contacting layer 26 and the second electrode 30 decreases, so that the operation voltage increases. Further, it has been found that the effective refractive index difference xcex94n in a lateral direction is small in the conventional GaN-based semiconductor laser device and is therefore not much effective for controlling the lateral mode, which causes the second problem. For overcoming the third problem, attention has been given to the fact that the number of steps of producing a nitride-based semiconductor laser device can be decreased by modifying the steps of forming and removing an etching mask for forming the ridge structure and forming and removing an etching mask for forming the mesa structure.
The nitride-based semiconductor laser device of the present invention for achieving the above object comprises;
(A) a first contacting layer formed on a substrate,
(B) a first electrode formed on the first contacting layer,
(C) a first cladding layer formed on the first contacting layer,
(D) an active layer formed on the first cladding layer,
(E) a second cladding layer formed on the active layer,
(F) a second contacting layer formed on the second cladding layer, and
(G) a second electrode formed on the second contacting layer,
the second cladding layer comprising a lower layer and an upper layer,
the first contacting layer, the first cladding layer, the active layer, the second cladding layer and the second contacting layer being composed of a nitride-based compound semiconductor layer each,
the first cladding layer, the active layer and the lower layer of the second cladding layer having a mesa structure,
the upper layer of the second cladding layer and the second contacting layer having a ridge structure having a smaller width than the mesa structure,
the second electrode having substantially the same width as the second contacting layer has in the interface of the second contacting layer and the electrode,
an insulating layer being formed on the portions of the lower layer of the second cladding layer which portions correspond to the top surface of the mesa structure, said insulating layer covering at least part of each of both side surfaces of the upper layer of the second cladding layer, and
a metal layer being formed on the top surface of the insulating layer and the top surface of the second electrode such that the metal layer continues from the top surface of the insulating layer to the top surface of the second electrode, said metal layer having substantially the same width as the mesa structure has.
In the index-guide type nitride-based semiconductor laser device of the present invention, the phrase xe2x80x9cthe metal layer having substantially the same width as the mesa structure hasxe2x80x9d means that the width of the mesa structure and the width of the metal layer are equal to each other within a tolerance of fluctuation dependent upon processing accuracy in the production steps of the nitride-based semiconductor laser device. Further, the phrase xe2x80x9cthe second electrode having substantially the same width as the second contacting layer has in the interface of the second contacting layer and the second electrodexe2x80x9d means that the second electrode has the same width as the second contacting layer has within a tolerance of fluctuation dependent upon processing accuracy in the production steps of the nitride-based semiconductor laser device. When the ridge structure has slanting side surfaces, the width of the ridge structure stands for the largest width of the ridge structure. That is, it refers to a width of the upper layer in the interface of the upper layer and the lower layer of the second cladding layer. When the laser beam travel direction of the nitride-based semiconductor laser device is taken as an X axis and when the thickness direction of the nitride-based semiconductor laser device (the direction of a normal line with regard to the substrate surface) is taken as a Z axis, the side surface of the mesa structure, the side surface of the ridge structure or the side surface of each layer means a surface that constitutes an outer surface thereof and crosses a Y axis. Further, the width of the mesa structure, the width of the ridge structure or the width of each layer means a length obtained along the Y-axis.
In the nitride-based semiconductor laser device of the present invention, there may be employed a constitution in which a protection layer is formed on the surface of the first contacting layer and is formed on the side surfaces of the mesa structure and the top surface of the metal layer, such that the protection layer continues from the first contacting layer through the side surfaces of the mesa structure to the top surface of the metal layer; a first opening portion is formed in a portion of the protection layer formed on the surface of the first contacting layer; the first electrode is formed on the first contacting layer exposed in the bottom of the first opening portion; a first pad electrode is formed on the first electrode; a second opening portion is formed in a portion of the protection layer on the metal layer on the second electrode; and a second pad electrode is formed on the exposed portion of the metal layer.
The method for the production of a nitride-based semiconductor laser device, provided by the present invention, for achieving the above object is a method for the production of a nitride-based semiconductor laser device comprising;
(A) a first contacting layer formed on a substrate,
(B) a first electrode formed on the first contacting layer,
(C) a first cladding layer formed on the first contacting layer,
(D) an active layer formed on the first cladding layer,
(E) a second cladding layer formed on the active layer,
(F) a second contacting layer formed on the second cladding layer, and
(G) a second electrode formed on the second contacting layer,
the second cladding layer comprising a lower layer and an upper layer,
the first contacting layer, the first cladding layer, the active layer, the second cladding layer and the second contacting layer being composed of a nitride-based compound semiconductor layer each,
the first cladding layer, the active layer and the lower layer of the second cladding layer having a mesa structure,
the upper layer of the second cladding layer and the second contacting layer having a ridge structure having a smaller width than the mesa structure,
the second electrode having substantially the same width as the second contacting layer has in the interface of the second contacting layer and the second electrode,
an insulating layer being formed on the portions of the lower layer of the second cladding layer which portions correspond to the top surface of the mesa structure, said insulating layer covering at least part of each of both side surfaces of the upper layer of the second cladding layer, and
a metal layer being formed on the top surface of the insulating layer and the top surface of the second electrode such that the metal layer continues from the top surface of the insulating layer to the top surface of the second electrode, said metal layer having substantially the same width as the mesa structure has,
said method comprising the steps of;
(a) consecutively depositing the first contacting layer, the first cladding layer, the active layer, the second cladding layer and the second contacting layer on the substrate, and then forming the second electrode having substantially the same width as the second contacting layer to be formed, on the second contacting layer,
(b) while using the second electrode as an etching mask, etching the second contacting layer and further partly etching the second cladding layer in the second cladding layer thickness direction, to form the second contacting layer and the upper layer of the second cladding layer having the ridge structure and to form the lower layer of the second cladding layer having top surfaces of which are exposed on both sides of the upper layer of the second cladding layer,
(c) forming the insulating layer on the portions of the lower layer of the second cladding layer on which portions no upper layer of the second cladding layer is formed, so as to cover at least part of each of both side surfaces of the upper layer of the second cladding layer and expose the top surface of the second electrode,
(d) forming the metal layer having substantially the same width as the width of the mesa structure to be formed, on the insulating layer and the top surface of the second electrode such that the metal layer continues from the surface of the insulating layer to the top surface of the second electrode, and
(e) while using the metal layer as an etching mask, etching at least the insulating layer, the lower layer of the second cladding layer, the active layer and the first cladding layer, to form the mesa structure.
In the method for the production of an index-guide type nitride-based semiconductor laser device, provided by the present invention, the phrase xe2x80x9cthe metal layer having substantially the same width as the mesa structure hasxe2x80x9d means that the width of the mesa structure and the width of the metal layer are equal to each other within a tolerance of fluctuation dependent upon processing accuracy in the production steps of the nitride-based semiconductor laser device. Further, the phrase xe2x80x9cthe second electrode having substantially the same width as the second contacting layer to be formedxe2x80x9d means that the second electrode has the same width as the second contacting layer has, in the interface of the second contacting layer and the second electrode, within a tolerance of fluctuation dependent upon processing accuracy in the production steps of the nitride-based semiconductor laser device. When the ridge structure has a slanting side surface, the width of the ridge structure stands for the largest width of the ridge structure. That is, it refers to a width of the upper layer in the interface of the upper layer and the lower layer of the second cladding layer. When the laser beam travel direction of the nitride-based semiconductor laser device is taken as an X axis and when the thickness direction of the nitride-based semiconductor laser device (the direction of a normal line with regard to the substrate surface) is taken as a Z axis, the side surface of the mesa structure, the side surface of the ridge structure or the side surface of each layer means a surface that constitutes an outer surface thereof and crosses a Y axis. Further, the width of the mesa structure, the width of the ridge structure or the width of each layer means a length obtained along the Y-axis.
In the method for the production of a nitride-based semiconductor laser device, provided by the present invention, in the step (e), preferably, the etching of the first cladding layer is followed by partly etching the first contacting layer in the thickness direction thereof.
The method for the production of a nitride-based semiconductor laser device, provided by the present invention, may have a constitution in which, after the step (e), the formation of a protection layer on the surface of the first contacting layer, the side surfaces of the mesa structure and the top surface of the metal layer, such that the protection layer continues from the surface of the first contacting layer through the side surfaces of the mesa structure to the top surface of the metal layer, is further followed by the steps of;
{circle around (1)} forming a first opening portion in a portion of the protection layer formed on the surface of the first contacting layer,
{circle around (2)} forming a first electrode on the exposed first contacting layer,
{circle around (3)} forming a first pad electrode on the first electrode,
{circle around (4)} forming a second opening portion in the protection layer on the metal layer on the second electrode, and
{circle around (5)} forming a second pad electrode on the exposed portion of the metal layer.
The order in which the above steps {circle around (1)} to {circle around (5)} are carried out includes the following orders. 
The material for the protection layer includes SiO2, SiNx, AlN, Al2O3, Ta2O5, ZrO2, ZnO, SiON, HfO2, Sc2O3, Y2O3 and MgO.
In the nitride-based semiconductor laser device of the present invention or the method for the production thereof, any insulating layer will do so long as it is formed on the portions of the lower layer of the second cladding layer on which portions no upper layer of the second cladding layer is formed, such that the insulating layer covers at least part of each of both side surfaces of the upper layer of the second cladding layer. Specifically, the embodiment of the insulating layer includes;
(1) an embodiment in which the insulating layer covers a lower portion of each of both side surfaces of the upper layer of the second cladding layer, and
(2) an embodiment in which the insulating layer covers at least each of both side surfaces of the upper layer of the second cladding layer.
More specifically, the embodiment (2) in which the insulating layer covers at least each of both side surfaces of the upper layer of the second cladding layer includes;
(2-1) an embodiment in which the insulating layer covers each of both side surfaces of the upper layer of the second cladding layer,
(2-2) an embodiment in which the insulating layer covers each of both side surfaces of the upper layer of the second cladding layer and further covers a lower portion of each of both side surfaces of the second contacting layer, and
(2-3) an embodiment in which the insulating layer covers each of both side surfaces of the upper layer of the second cladding layer and further covers each of both side surfaces of the second contacting layer.
Desirably, the above insulating layer formed on the portions of the lower layer of the second cladding layer on which portions no upper layer of the second cladding layer is formed, other than vicinities of both side surfaces of the upper layer of the second cladding layer has generally uniform thickness.
In the method for the production of a nitride-based semiconductor laser device, provided by the present invention, the insulating layer can be formed depending upon the above various embodiments in the above step (c). That is, for example, in the embodiment in which the insulating layer covers at least each of both side surfaces of the upper layer of the second cladding layer, it is sufficient to form the insulating layer on the portions of the lower layer of the second cladding layer on which portions no upper layer of the second cladding layer is formed, such that the insulating layer covers at least each of both side surfaces of the upper layer of the second cladding layer, in the step (c).
In the embodiment in which the insulating layer covers a lower portion of each of both side surfaces of the upper layer of the second cladding layer in the nitride-based semiconductor laser device of the present invention or the method for the production thereof, the metal layer is in contact with an upper portion of each of both side surfaces of the upper layer of the second cladding layer and electric current is injected into the upper layer of the second cladding layer, while no problem is caused. In the above embodiment, however, the insulating layer formed on the portions of the lower layer of the second cladding layer on which portions no upper layer of the second cladding layer is formed has a small thickness, so that light emitted in the active layer may undergo loss, or that a dielectric breakdown may occur between the active layer and the metal layer. It is therefore preferred to employ the embodiment in which the insulating layer covers at least each of both side surfaces of the upper layer of the second cladding layer. When the above embodiment is employed, a dielectric breakdown between the active layer and the metal layer can be reliably prevented even if the thickness of the upper layer of the second cladding layer is increased and even if the thickness of the portions of the lower layer of the second cladding layer on which portions no upper layer of the second cladding layer is formed is decreased. Further, the effective refractive index difference xcex94n in a lateral direction can be fully increased, and the stability of the lateral mode can be further improved.
In the method for the production of a semiconductor laser device provided by the present invention, in the above step (c), preferably, a photoresist film is formed on the insulating layer after the formation of the insulating layer on the entire surface, such that the photoresist film has a smaller thickness above the second electrode and has a larger thickness above the portions of the lower layer of the second cladding layer on which portions no upper layer of the second cladding layer is formed, and then the photoresist film and the insulating layer at least on the second electrode are etched to expose at least the top surface (contact surface to the metal layer) of the second electrode. That is, preferably, a height difference between the second electrode and the lower layer of the second cladding layer is utilized, and the second electrode is allowed to work as an etching stop layer, thereby to expose at least the top surface (contact surface to the metal layer) of the second electrode. By the above procedure, a large-thickness insulating layer can be formed on both side surfaces of the upper layer of the second cladding layer having a ridge structure.
In the nitride-based semiconductor laser device of the present invention including the above preferred embodiments or the method for the production thereof, the thickness TINSL of a portion of the insulating layer formed on the portions of the lower layer of the second cladding layer which portions correspond to the top surface of the mesa structure (more specifically, thickness of the insulating layer formed on the portions of the lower layer of the second cladding layer on which portions no upper layer of the second cladding layer is formed, other than vicinities of both side surfaces of the upper layer of the second cladding layer) is 5xc3x9710xe2x88x928 m to 3xc3x9710xe2x88x927 m, preferably 9xc3x9710xe2x88x928 m to 2xc3x9710xe2x88x927 m, in view of the effective refractive index at which a light confinement effect can be exhibited.
Alternatively, in the nitride-based semiconductor laser device of the present invention including the above preferred various embodiments or the method for the production thereof, it is desirable to satisfy 0.4TTOTALxe2x89xa6TUPPERxe2x89xa60.9TTOTAL, preferably 0.5TTOTALxe2x89xa6TUPPERxe2x89xa60.8TTOTAL, in which TTOTAL is a total thickness of the second cladding layer and TUPPER is a thickness of the upper layer of the second cladding layer. More specifically, TTOTAL is 5xc3x9710xe2x88x927 m to 1xc3x9710xe2x88x926 m, preferably 6xc3x9710xe2x88x927 m to 8xc3x9710xe2x88x927 m, and TUPPER is 2xc3x9710xe2x88x927 m to 9xc3x9710xe2x88x927 m, preferably 3xc3x9710xe2x88x927 m to 6.4xc3x9710xe2x88x927 m. In this case, it is desirable to satisfy 0.05TUPPERxe2x89xa6TINSL, preferably 0.1TUPPERxe2x89xa6TINSL in which TINSL is a thickness of the portion of the insulating layer which portion is formed on the portions of the lower layer of the second cladding layer which portions correspond to the top surface of the mesa structure (more specifically, a thickness of the insulating layer formed on the portions of the lower layer of the second cladding layer on which portions no upper layer of the second cladding layer is formed, other than vicinities of both side surfaces of the upper layer of the second cladding layer).
In the nitride-based semiconductor laser device of the present invention including the above preferred various embodiments or the method for the production thereof, desirably, the insulating layer is made of at least one material selected from the group consisting of SiO2, SiNx, AlN, Al2O3, Ta2O5 and ZrO2. The insulating layer may have a single-layered structure or a multi-layered structure composed of these material(s). For preventing damage of the exposed portion of the lower layer of the second cladding layer during the formation of the insulating layer, desirably, the insulating layer made of SiO2, SiNx, Al2O3 or ZrO2 is formed by a vacuum deposition method, or the insulating layer made of AlN, Al2O3, Ta2O5 or ZrO2 is formed by a sputtering method, although the method for forming the insulating layer shall not be limited thereto. In some cases, a silicon layer (specifically, for example, an amorphous silicon layer) may be formed on the insulating layer (specifically, between the insulating layer and the metal layer), for example, by a vacuum deposition method or some other method. The silicon layer works as a light-absorbing layer, and when the silicon layer is formed, absorption to primary-mode light increases, so that the lateral mode can be stabilized.
When the insulating layer is made of SiO2, desirably, the thickness TINSL of the insulating layer made of SiO2 is 2xc3x9710xe2x88x928 m to 3xc3x9710xe2x88x927 m, preferably 2xc3x9710xe2x88x928 m to 2xc3x9710xe2x88x927 m. When the silicon layer is formed on the insulating layer made of SiO2, the thickness TINSL of the insulating layer made of SiO2 is 2xc3x9710xe2x88x928 m to 8xc3x9710xe2x88x928 m, and the thickness of the silicon layer is at least 5xc3x9710xe2x88x929 m. Preferably, the thickness TINSL of the insulating layer made of SiO2 is 4xc3x9710xe2x88x928 m to 8xc3x9710xe2x88x928 m, and the thickness of the silicon layer is at least 5xc3x9710xe2x88x929 m. More preferably, the thickness TINSL of the insulating layer made of SiO2 is 4xc3x9710xe2x88x928 m to 8xc3x9710xe2x88x928 m, and the thickness of the silicon layer is at least 2xc3x9710xe2x88x928 m.
When the insulating layer is made of SiNx, desirably, the thickness TINSL of the insulating layer made of SiNx is 2xc3x9710xe2x88x928 m to 3xc3x9710xe2x88x927 m, preferably 2xc3x9710xe2x88x928 m to 2xc3x9710xe2x88x927 m. When the silicon layer is formed on the insulating layer made of SiNx, the thickness TNSL of the insulating layer made of SiNx is 2xc3x9710xe2x88x928 m to 2xc3x9710xe2x88x927 m, and the thickness of the silicon layer is at least 5xc3x9710xe2x88x929 m. Preferably, the thickness TINSL of the insulating layer made of SiNx is 5xc3x9710xe2x88x928 m to 8xc3x9710xe2x88x928 m, and the thickness of the silicon layer is at least 5xc3x9710xe2x88x929 m. More preferably, the thickness TINSL of the insulating layer made of SiNx is 5xc3x9710xe2x88x928 m to 8xc3x9710xe2x88x928 m, and the thickness of the silicon layer is at least 2xc3x9710xe2x88x928 m.
When the insulating layer is made of AlN, desirably, the thickness TINSL of the insulating layer made of AlN is 2xc3x9710xe2x88x928 m to 3xc3x9710xe2x88x927 m, preferably 2xc3x9710xe2x88x928 m to 2xc3x9710xe2x88x927 m. When the silicon layer is formed on the insulating layer made of AlN, the thickness TINSL of the insulating layer made of AlN is 2xc3x9710xe2x88x928 m to 2xc3x9710xe2x88x927 m, and the thickness of the silicon layer is at least 5xc3x9710xe2x88x929 m. Preferably, the thickness TINSL of the insulating layer made of AlN is 5xc3x9710xe2x88x928 m to 1xc3x9710xe2x88x927 m, and the thickness of the silicon layer is at least 5xc3x9710xe2x88x929 m. More preferably, the thickness TINSL of the insulating layer made of AlN is 5xc3x9710xe2x88x928 m to 1xc3x9710xe2x88x927 m, and the thickness of the silicon layer is at least 2xc3x9710xe2x88x928 m.
When the insulating layer is made of Al2O3, desirably, the thickness TINSL of the insulating layer made of Al2O3 is 2xc3x9710xe2x88x928 m to 3xc3x9710xe2x88x927 m, preferably 2xc3x9710xe2x88x928 m to 2xc3x9710xe2x88x927 m. When the silicon layer is formed on the insulating layer made of Al2O3, the thickness TINSL of the insulating layer made of Al2O3 is 2xc3x9710xe2x88x928 m to 1.0xc3x9710xe2x88x927 m, and the thickness of the silicon layer is at least 5xc3x9710xe2x88x929 m. Preferably, the thickness TINSL of the insulating layer made of Al2O3 is 4xc3x9710xe2x88x928 m to 1xc3x9710xe2x88x927 m, and the thickness of the silicon layer is at least 5xc3x9710xe2x88x929 m. More preferably, the thickness TINSL of the insulating layer made of Al2O3 is 4xc3x9710xe2x88x928 m to 1xc3x9710xe2x88x927 m, and the thickness of the silicon layer is at least 2xc3x9710xe2x88x928 m.
When the insulating layer is made of Ta2O5, desirably, the thickness TINSL of the insulating layer made of Ta2O5 is 2xc3x9710xe2x88x928 m to 5xc3x9710xe2x88x927 m, preferably 2xc3x9710xe2x88x928 m to 4xc3x9710xe2x88x927 m. When the silicon layer is formed on the insulating layer made of Ta2O5, the thickness TINSL of the insulating layer made of Ta2O5 is 2xc3x9710xe2x88x928 m to 2xc3x9710xe2x88x927 m, and the thickness of the silicon layer is at least 5xc3x9710xe2x88x929 m. Preferably, the thickness TINSL of the insulating layer made of Ta2O5 is 2xc3x9710xe2x88x928 m to 1xc3x9710xe2x88x927 m, and the thickness of the silicon layer is at least 5xc3x9710xe2x88x929 m. More preferably, the thickness TINSL of the insulating layer made of Ta2O5 is 2xc3x9710xe2x88x928 m to 1xc3x9710xe2x88x927 m, and the thickness of the silicon layer is at least 2xc3x9710xe2x88x928 m.
When the insulating layer is made of ZrO2, desirably, the thickness TINSL of the insulating layer made of ZrO2 is 2xc3x9710xe2x88x928 m to 3xc3x9710xe2x88x927 m, preferably 2xc3x9710xe2x88x928 m to 2xc3x9710xe2x88x927 m. When the silicon layer is formed on the insulating layer made of ZrO2, the thickness TINSL of the insulating layer made of ZrO2 is 2xc3x9710xe2x88x928 m to 2xc3x9710xe2x88x927 m, and the thickness of the silicon layer is at least 5xc3x9710xe2x88x929 m. Preferably, the thickness TINSL of the insulating layer made of ZrO2 is 3xc3x9710xe2x88x928 m to 1.1xc3x9710xe2x88x927 m, and the thickness of the silicon layer is at least 5xc3x9710xe2x88x929 m. More preferably, the thickness TINSL of the insulating layer made of ZrO2 is 6xc3x9710xe2x88x928 m to 1.1xc3x9710xe2x88x927 m, and the thickness of the silicon layer is at least 2xc3x9710xe2x88x928 m.
Alternatively, the material for the insulating layer may be at least one material selected from the group consisting of ZnO, SiON, HfO2, Sc2O3, Y2O3, MgO, ThO2 and Bi2O3. The insulating layer may have a single-layered structure or a multi-layered structure composed of these material(s). Further, the insulating layer may have a multi-layered structure composed of a combination of these material(s) and the above-described material(s). The insulating layer may be replaced, for example, with a first-conduction type AlxGa1xe2x88x92xN (Xxe2x89xa70.02) layer that works as a current confinement layer. In this case, the second cladding layer is of a second-conduction type. That is, the conduction type of the second cladding layer is p-type, an n-type type AlxGa1xe2x88x92xN (Xxe2x89xa70.02) layer can be formed.
In the nitride-based semiconductor laser device of the present invention including the above preferred various embodiments or the method for the production thereof, desirably, the second electrode as an ohmic contact electrode to the second contacting layer has a single-layered structure or a multi-layered structure containing at least one metal selected from the group consisting of palladium (Pd), platinum (Pt), nickel (Ni) and gold (Au), and the metal layer has a single-layered structure or a multi-layered structure containing at least one metal selected from the group consisting of platinum (Pt), titanium (Ti) and nickel (Ni). The thickness of the second electrode is preferably 1xc3x9710xe2x88x928 m to 1xc3x9710xe2x88x926 m. The thickness of the metal layer is preferably 5xc3x9710xe2x88x928 m to 5xc3x9710xe2x88x927 m. Since the metal layer is employed, a high selective etching ratio to the nitride-based compound semiconductor layer can be obtained when the metal layer is used as an etching mask. Further, the metal layer works as a light absorption layer, and when the metal layer is formed, light absorption to a higher mode can be increased, and the lateral mode can be stabilized.
Specifically, when the second electrode has a single-layered structure composed, for example, of a 0.05 xcexcm thick Pd (palladium) layer, the adhesion of the second electrode to the second contacting layer can be particularly improved, and Pd attracts nitrogen atoms inside the second contacting layer to remove nitrogen vacancies in the second contacting layer immediately under the second electrode and, further, forms an hydrogen occlusion alloy, so that hydrogen is taken away from the second contacting layer containing, for example, a p-type impurity, whereby the p-type second contacting layer having a high carrier concentration can be obtained by activating the p-type impurity (p-type dopant). Further, when the second electrode has a single-layered structure composed, for example, of a 0.1 xcexcm thick Pt (platinum) layer, the diffusion of tin (Sn) atoms in a solder into the second contacting layer can be particularly prevented when the solder is used for electrically connecting the second electrode to an outside electrode or circuit. Alternatively, the second electrode may have a single-layered structure composed of an alloy containing Ni (nickel) or gold (Au). Further, the second electrode may have a multi-layered structure such as a Pd/Pt, Pd/Ni, Pd/Au, Pt/Pd, Pt/Ni, Pt/Au, Ni/Pd, Ni/Pt or Ni/Au multi-layered structure. In the above multi-layered structure, a material before xe2x80x9c/xe2x80x9d constitutes a lower layer, a material after xe2x80x9c/xe2x80x9d constitutes an upper layer, and xe2x80x9c/xe2x80x9d in a multi-layered structure will be used in this sense hereinafter.
When the metal layer has a single-layered structure composed, for example, of a 0.1 xcexcm thick Pt (platinum) layer, the diffusion of tin (Sn) atoms in a solder into the second contacting layer can be particularly prevented when the solder is used for electrically connecting the second electrode to an outside electrode or circuit. Further, when the metal layer has a single-layered structure composed, for example, of a 10 nm thick Ti (titanium) layer or a 0.1 xcexcm thick Ni (nickel) layer, the adhesion of the metal layer to the insulating layer can be particularly improved. Further, the metal layer may have a multi-layered structure such as a Ti/Pt, Ti/Ru, Ti/Rh, Ti/Os, Ti/Ir, Ti/Ag, Ti/Ni, Ti/Pt, Ti/Pt/Ni, Ni/Pt, Ni/Ru, Ni/Rh, Ni/Os, Ni/Ir or Ni/Ag multi-layered structure.
The first electrode as an ohmic contact electrode to the first contacting layer desirably has a single-layered structure or a multi-layered structure containing at least one metal selected from the group consisting of gold (Au), Al (aluminum), Ti (titanium), tungsten (W), Cu (copper), Zn (zinc), tin (Sn) and indium (In). Examples thereof include Ti/Al and Ti/Pt/Au. When the first electrode has a multi-layered structure composed of Ti/Pt/Au, for example, a Ti layer is 5 to 10 nm thick, a Pt layer is 1xc3x9710xe2x88x927 m thick, and an Au layer is 2xc3x9710xe2x88x927 m to 3xc3x9710xe2x88x927 m thick.
The second pad electrode formed on the metal layer desirably has a single-layered structure or a multi-layered structure containing at least one metal selected from the group consisting of Ti (titanium), Pt (platinum) and Au (gold). When the second pad electrode has a single-layered structure composed, for example, of a 10 nm thick Ti (titanium) layer, the adhesion of the second pad electrode to the metal layer can be particularly improved. When the second pad electrode has a single-layered structure composed, for example, of a 0.1 xcexcm thick Pt (platinum) layer, the diffusion of tin (Sn) atoms in a solder into the second contacting layer can be particularly prevented when the solder is used for electrically connecting the second electrode to an outside electrode or circuit. Further, when the metal layer has a single-layered structure composed, for example, of a 0.3 xcexcm thick Au (gold) layer, an alloy with tin (Sn) atoms in a solder can be formed when the solder is used for electrically connecting the second electrode to an outside electrode or circuit. The second pad electrode may have a multi-layered structure such as a Ti/Pt/Au or Ti/Au multi-layered structure.
For a combination of the materials for constituting the second electrode, the metal layer and the second pad electrode, preferably, the second electrode has one structure of the following six cases;
a single-layered structure of Pd,
a single-layered structure of Pt,
a single-layered structure of Ni,
a multi-layered structure of Pd/Pt,
a multi-layered structure of Pd/Ni, and
a multi-layered structure of Pd/Au,
the metal layer has one structure of the following 25 cases;
a multi-layered structure of Ti/Pt,
a multi-layered structure of Ti/Ru,
a multi-layered structure of Ti/Ru/Ni,
a multi-layered structure of Ti/Rh,
a multi-layered structure of Ti/Rh/Ni,
a multi-layered structure of Ti/Os,
a multi-layered structure of Ti/Os/Ni,
a multi-layered structure of Ti/Ir,
a multi-layered structure of Ti/Ir/Ni,
a multi-layered structure of Ti/Ag,
a multi-layered structure of Ti/Ag/Ni,
a multi-layered structure of Ti/Ni,
a multi-layered structure of Ti/Pt/Ni,
a multi-layered structure of Ni/Pt,
a multi-layered structure of Ni/Pt/Ni,
a multi-layered structure of Ni/Ru,
a multi-layered structure of Ni/Ru/Ni,
a multi-layered structure of Ni/Rh,
a multi-layered structure of Ni/Rh/Ni,
a multi-layered structure of Ni/Os,
a multi-layered structure of Ni/Os/Ni,
a multi-layered structure of Ni/Ir,
a multi-layered structure of Ni/Ir/Ni,
a multi-layered structure of Ni/Ag, and
a multi-layered structure of Ni/Ag/Ni, and
the second pad electrode has one structure of the following three cases;
a single-layered structure of Au,
a multi-layered structure of Ti/Au, and
a multi-layered structure of Ti/Pt/Au.
That is, the number of combinations of the materials for constituting the second electrode, the metal layer and the second pad electrode is 450 cases (6xc3x9725xc3x973=450). The combination of the materials may be any one of these cases. Of these combinations, a combination of materials of (Pd:Pt:Au) and a combination of (Pd/Pt:Ti/Pt/Ni:Ti/Pt/Au) are more preferred as a combination for constituting (second electrode:metal layer: second pad electrode).
In the nitride-based semiconductor laser device of the present invention or the method for the production thereof, desirably, the width of the ridge structure is at least 1.0 xcexcm but not more than 2.0 xcexcm, from the viewpoint of decreasing the power consumption of the nitride-based semiconductor laser device.
The first pad electrode formed on the first electrode desirably has a single-layered structure or a multi-layered structure containing at least one metal selected from the group consisting of Ti (titanium), Pt (platinum) and Au (gold).
In the present invention, the nitride-based compound semiconductor includes III-V group compound semiconductors containing a nitrogen element as a V group compound, such as GaN, an AlGaN mixed crystal, an AlInGaN mixed crystal, a BAlInGaN mixed crystal, an InGaN mixed crystal, InN and AlN. A nitride-based compound semiconductor layer can be deposited or formed, for example, by a metal organic chemical vapor deposition method (MOCVD method), a molecular beam epitaxy method (MBE method), or a hydride gas phase growth method in which a halogen contributes to transportation or a reaction. In the nitride-based semiconductor laser device of the present invention, or a nitride-based semiconductor laser device produced by the method for the production of a nitride-based semiconductor laser device, provided by the present invention, the nitride-based compound semiconductor is not specially limited in kind and composition, nor is the nitride-based compound semiconductor layer limited in structure and constitution, so long as the nitride-based semiconductor laser device has a stacked structure of the nitride-based compound semiconductor layers as a laser structure.
The substrate includes a sapphire substrate having a c-surface as a main surface, a GaN substrate and an SiC substrate.
In the present invention, the second contacting layer and the second cladding layer contain a p-type impurity, and the first electrode, the first contacting layer and the first cladding layer contain an n-type impurity. Alternatively, in the present invention, the second contacting layer and the second cladding layer contain an n-type impurity, and the first electrode, the first contacting layer and the first cladding layer contain a p-type impurity. The p-type impurity includes Mg, Zn, Cd, Be, Ca, Ba and O, and the n-type impurity includes Si, Ge, Se, Sn, C and Ti.
The plane form of the ridge structure includes the form of a stripe, the form of a taper and the form of a flare.
The nitride-based semiconductor laser device of the present invention has what can be called a shallow buried structure in which the insulating layer holds, from sides, the upper layer of the second cladding layer which upper layer forms the ridge structure. That is, the upper layer of the second cladding layer can have a large thickness, and the lower layer of the second cladding layer can have a small thickness. Further, the insulating layer is formed. Therefore, the nitride-based semiconductor laser device has a large current confinement effect and has excellent light output-current injection properties, and it has a sufficiently large effective refractive index difference xcex94n in a lateral direction, so that it has high controllability to a lateral mode and that it is highly stable in a lateral mode. Even if the upper layer of the second cladding layer is increased in thickness and even if the lower layer of the second cladding layer is decreased in thickness, an insulating layer having a sufficient thickness can be formed, so that there is no possibility of leak current flowing from the second pad electrode through the insulating layer and the lower layer of the second cladding layer.
In the method for the production of a nitride-based semiconductor laser device, provided by the present invention, the second-electrode is formed on the second cladding layer in a step that follows the step of depositing the second contacting layer, so that contamination of the top surface of the second contacting layer (contact surface to the second electrode) is suppressed, and that the deviation of the operation voltage from a desired value or a designed value can be prevented. Further, since the second electrode is formed on the second contacting layer before the formation of the insulating layer, no damage is caused on the top surface of the second contacting layer (contact surface to the second electrode) even if any method is employed to form the insulating layer. Further, the second contacting layer is etched by a self-alignment manner using the second electrode as an etching mask, and the second cladding layer is etched partly in its thickness direction to form the ridge structure, so that the second electrode can be formed on the second cladding layer such that the second electrode has substantially the same form and dimensions as those of the top surface of the second contacting layer (contact surface to the second electrode) and that no positional deviation (displacement) takes place between the second electrode and the second cladding layer unlike any conventional technique. Moreover, since the second electrode is used as an etching mask for forming the ridge structure, the steps of forming an etching mask for forming a ridge structure and removing the same are no longer required. Since the metal layer is used as an etching mask for forming the mesa structure, the steps of forming an etching mask for forming a mesa structure and removing the same are no longer required. Therefore, the number of the steps for the production process of the nitride-based semiconductor laser device is small as compared with any conventional production method, which can achieve an improvement in productivity.